Aerosols

UXL Encyclopedia of Science
COPYRIGHT 2002 The Gale Group, Inc.

Aerosols

Aerosols are collections of tiny particles of solid and/or liquid suspended in a gas. The size of particles in an aerosol ranges from about 0.001 to about 100 microns. (A micron is one-millionth of a meter.) The most familiar form of an aerosol is the pressurized spray can, which can dispense anything from hair spray to enamel paint to whipping cream. Aerosols are produced by a number of natural processes and are now manufactured in large quantities for a variety of commercial uses. They are also at the root of a number of environmental problems, including air pollution and destruction of ozone, a natural component of Earth's atmosphere.

Classification

Aerosols are commonly classified into various subgroups based on the nature and size of the particles of which they are composed and, to some extent, the manner in which the aerosol is formed. Although relatively strict scientific definitions are available for each subgroup, these distinctions may become blurred in actual practical applications. The most important of these subgroups are fumes, dusts, mists, and sprays.

Fumes. Fumes consist of solid particles—ranging in size from 0.001 to 1 micron—suspended in a gas. Probably the most familiar form of a fume is smoke. Smoke is formed from the incomplete combustion of fuels such as coal, oil, or natural gas. The particles that make up smoke are smaller than 10 microns in size.

Dusts. Dusts also contain solid particles suspended in a gas, usually air, but the particles are larger in size than those in a fume. They range from about 1 to about 100 microns in size, although they may be even larger. Dust is formed by the release of materials such as soil and sand, fertilizers, coal dust, cement dust, pollen, and fly ash into the atmosphere. Because of their larger particle size, dusts tend to be more unstable and settle out more rapidly than do fumes, which do not settle out at all.

Mists. Mists are liquid particles—less than about 10 microns in size—dispersed in a gas. The most common type of mist is that formed by tiny water droplets suspended in the air, as on a cool summer morning. If the concentration of liquid particles becomes high enough to affect visibility, it is then called a fog. A particular form of fog that has become significant in the last half century is smog. Smog forms when natural moisture in the air interacts with human-produced components, such as smoke and other combustion products, to form chemically active materials.

Sprays. Sprays form when relatively large (10+ microns) droplets of a liquid are suspended in a gas. Sprays can be formed naturally, as along an ocean beach, but are also produced as the result of some human inventions such as aerosol can dispensers of paints, deodorants, and other household products.

Sources

About three-quarters of all aerosols found in Earth's atmosphere come from natural sources. The most important of these natural components are sea salt, soil and rock debris, products of volcanic emissions, smoke from forest fires, and solid and liquid particles formed by chemical reactions in the atmosphere.

Volcanic eruptions are major, if highly irregular, sources of atmospheric aerosols. The eruptions of Mount Hudson in Chile in August 1991 and Mount Pinatubo in the Philippines in June 1991 produced huge volumes of aerosols that had measurable effects on Earth's atmosphere.

Words to Know

Acid rain: A form of precipitation that is significantly more acidic than neutral water, often produced as the result of industrial processes.

Chlorofluorocarbons (CFCs): A group of organic compounds once used widely as propellants in commercial sprays but regulated in the United States since 1987 because of their harmful environmental effects.

Dust: An aerosol consisting of solid particles in the range of 1 to 100 microns suspended in a gas.

Electrostatic precipitator: A device for removing pollutants from a smokestack.

Fume: A type of aerosol consisting of solid particles in the range 0.001 to 1 micron suspended in a gas.

Micron: One-millionth of a meter.

Mist: A type of aerosol consisting of droplets of liquid less than 10 microns in size suspended in a gas.

Ozone layer: A region of the upper atmosphere in which the concentration of ozone is significantly higher than in other parts of the atmosphere.

Smog: An aerosol form of air pollution produced when moisture in the air combines and reacts with the products of fossil fuel combustion.

Smoke: A fume formed by the incomplete combustion of fossil fuels such as coal, oil, and natural gas.

Spray: A type of aerosol consisting of droplets of liquid greater than 10 microns in size suspended in a gas.

Stack gases: Gases released through a smokestack as the result of some power-generating or manufacturing process.

The remaining atmospheric aerosols result from human actions. Some, such as the aerosols released from spray-can products, go directly to form aerosols in the atmosphere. Others undergo chemical changes; for example, oxides of nitrogen and sulfur are produced during the combustion of fossil fuels such as coal and oil. These oxides may be converted to liquid or solid nitrates and sulfates, which are then incorporated into atmospheric aerosols.

Physical properties

The physical and chemical properties of an aerosol depend to a large extent on the size of the particles that make it up. When those particles are very large, they tend to have the same properties as a macroscopic (large size) sample of the same material. The smaller the particles are, however, the more likely they are to take on new characteristics different from those of the same material in bulk.

Aerosols tend to coagulate, or to collide and combine with each other to form larger bodies. A cloud, for example, consists of tiny droplets of water and tiny ice crystals. These particles move about randomly within the cloud, colliding with each other from time to time. As a result of a collision, two water particles may adhere (stick) to each other and form a larger, heavier particle. This process results in the formation of droplets of water or crystals of ice heavy enough to fall to Earth as rain, snow, or some other form of precipitation.

Synthetic production

The synthetic production of aerosols for various commercial purposes has become such a large industry that the term aerosol has taken on a new meaning. Average citizens who know little or nothing about the scientific aspects of aerosols recognize the term as referring to devices for dispensing a wide variety of products, including hair spray, furniture polish, and spray paint.

The concept of aerosol technology is relatively simple. A spray can is filled with a product to be delivered (such as paint), a propellant, and sometimes a carrier to help disperse the product. Pressing a button on the can releases a mixture of these components in the form of an aerosol.

Most aerosols, however, are actually more complex than this simple description. An aerosol pesticide, for example, must be formulated in such a way that a precise amount of poison is released, enough to kill pests, but not so much as to produce an environmental hazard. Similarly, an aerosol throat spray must deliver a carefully measured quantity of medication.

The production of commercial aerosols fell slightly in the late 1980s because of concerns about the ozone (see Environmental factors below) and other environmental effects. By 1992, however, their manufacture had rebounded. In that year 990 million container units (bottles and cans) of personal aerosol products and 695 million container units of household products were manufactured.

Combustion aerosols

Aerosol technology has made possible vastly improved combustion systems, such as those used in fossil-fueled power generator plants and in rocket engines. The fundamental principle involved is that any solid or liquid fuel burns only at its surface. The combustion of a lump of coal proceeds relatively slowly because inner parts of the coal cannot begin to burn until the outer layers are burned off first.

The rate of combustion can be increased by dividing a lump of coal or a barrel of fuel oil into very small particles, the smaller the better. Power-generating plants today often run on coal that has been pulverized to a dust or on oil that has been converted to a mist. The dust or mist is then thoroughly mixed with an oxidizing agent, such as air or pure oxygen, and fed into the combustion chamber. The rate of combustion of such aerosols is many times greater than would be the case for coal or oil in bulk.

Environmental factors

A number of environmental problems have been connected to aerosols, the vast majority of them associated with aerosols produced by human activities. For example, smoke released during the incomplete combustion of fossil fuels results in the formation of at least two major types of aerosols that may be harmful to plant and animal life. One type consists of finely divided carbon released from unburned fuel. This soot can damage plants by coating their leaves and reducing their ability to carry out photosynthesis (using light to break down chemical compounds). It can also clog the alveoli—air sacs in human lungs—and interfere with a person's respiration.

A second type of harmful aerosol is formed when smokestack gases, such as sulfur dioxide and nitrogen oxides, react with oxygen and water vapor in the air to form sulfuric and nitric acids, respectively. Mists containing these acids may be carried hundreds of miles from their original source before conglomeration occurs and the acids fall to Earth as acid rain. Considerable disagreement exists about the precise nature and extent of the damage caused by acid rain. But there seems to be little doubt that in some locations it has caused severe harm to plant and aquatic life.

Ozone depletion. A particularly serious environmental effect of aerosol technology has been damage to Earth's ozone layer. This damage appears to be caused by a group of compounds known as chlorofluorocarbons (CFCs) which, for more than a half century, were by far the most popular of all propellants used in aerosol cans.

Scientists originally felt little concern about the use of CFCs in aerosol products because they are highly stable compounds at conditions encountered on Earth's surface. They have since learned, however, that CFCs behave very differently when they diffuse into the upper atmosphere and are exposed to the intense solar radiation present there.

Under those circumstances, CFCs decompose and release chlorine atoms that, in turn, react with ozone in the stratosphere (the atmospheric region approximately 7 to 31 miles above Earth's surface). As a result, the concentration of ozone in portions of the atmosphere has been steadily decreasing. This change could prove to be very dangerous, since Earth's ozone layer absorbs ultraviolet radiation from the Sun and protects living things on our planet from the harmful effects of that radiation.

Technological solutions. Methods for reducing the harmful environmental effects of aerosols such as those described above have received the serious attention of scientists for many years. Consequently, a number of techniques have been invented for reducing the aerosol components of things like stack gases. One device, the electrostatic precipitator, is based on the principle that the particles of which an aerosol consists (such as unburned carbon in stack gases) carry small electrical charges. By lining a smokestack with charged metal grids, the charged aerosol particles can be attracted to the grids and pulled out of the emitted smoke.

Efforts aimed at solving the CFC/ozone problem have not yet been as successful as those used to combat other forms of air pollution. Chemists have developed and tested a number of substitutes for CFCs as aerosol propellants. One group of special interest has been the hydrochlorofluorocarbons (HCFCs), CFC-like compounds that also contain hydrogen atoms. Most CFC-substitutes tried so far, however, are very expensive, not efficient enough as a propellant, or equally harmful to the environment.

The dangers of aerosol sniffing

Another risk associated with commercial aerosols is their use as recreational drugs. Inhalation of some consumer aerosol preparations may produce a wide variety of effects, including euphoria, excitement, delusions, and hallucinations. Repeated sniffing of aerosols can result in addiction that can cause intoxication, damaged vision, slurred speech, and diminished mental capacity.

[See alsoAcid rain; Pollution; Ozone ]

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aerosol

aerosol A colloidal substance, either natural or man-made, that is suspended in the air because the small size (0.01–10 μm) of its particles makes them fall slowly. Aerosols in the troposphere are usually removed by precipitation and their residence time is measured in days or weeks. Aerosols that are carried into the stratosphere usually remain there much longer. Tropospheric aerosols may act as Aitken nuclei but the general effect of aerosols is to absorb, reflect, or scatter radiation. Stratospheric aerosols, mainly sulphate particles resulting from volcanic eruptions, may reduce insolation significantly. About 30 per cent of tropospheric dust particles are the result of human activities. See atmospheric structure; Mie scattering; Rayleigh scattering; and volcanic dust.

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aerosol

aerosol Colloidal substance, either natural or manmade, that is suspended in the air because the small size (0.01–10 μm) of its particles makes them fall slowly. Aerosols in the troposphere are usually removed by precipitation and their residence time is measured in days or weeks. Aerosols that are carried into the stratosphere usually remain there much longer. Tropospheric aerosols may act as Aitken nuclei but the general effect of aerosols is to absorb, reflect, or scatter radiation. Stratospheric aerosols, mainly sulphate particles resulting from volcanic eruptions, may reduce insolation significantly. About 30% of tropospheric dust particles are the result of human activities. See ATMOSPHERIC STRUCTURE; MIE SCATTERING; RAYLEIGH SCATTERING; and VOLCANIC DUST.

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aerosol

aerosol Suspension of liquid or solid particles in a gas. Fog – millions of tiny water droplets suspended in air – is a liquid-based example; airborne dust or smoke is a solid-based equivalent. Manufactured aerosols are used in products such as deodorants, cosmetics, paints and household sprays. Chlorofluorocarbons(CFCs) are being phased out as aerosol propellants because they damage the ozone layer.

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aerosol

aer·o·sol
/ ˈerəˌsôl; -ˌsäl/
•
n.
a substance enclosed under pressure and able to be released as a fine spray, typically by means of a propellant gas. ∎
a container holding such a substance.
∎ Chem.
a colloidal suspension of particles dispersed in air or gas.

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Aerosol

Environmental Encyclopedia
COPYRIGHT 2003 The Gale Group Inc.

Aerosol

A suspension of particles, liquid or solid, in a gas. The term implies a degree of permanence in the suspension, which puts a rough upper limit on particle size at a few tens of micrometers at most (1 micrometer = 0.00004 in). Thus in proper use the term connotes the ensemble of the particles and the suspending gas.

The atmospheric aerosol has two major components, generally referred to as coarse and fine particles, with different sources and different composition. Coarse particles result from mechanical processes, such as grinding. The smaller particles are ground, the more surface they have per unit of mass. Creating new surface requires energy, so the smallest average size that can be created by such processes is limited by the available energy. It is rare for such mechanically generated particles to be less than 1 μm (0.00004 in.) in diameter. Fine particles, on the other hand, are formed by condensation from the vapor phase. For most substances, condensation is difficult from a uniform gaseous state; it requires the presence of pre-existing particles on which the vapors can deposit. Alternatively, very high concentrations of the vapor are required, compared with the concentration in equilibrium with the condensed material.

Hence, fine particles form readily in combustion processes when substances are vaporized. The gas is then quickly cooled. These can then serve as nuclei for the formation of larger particles, still in the fine particle size range, in the presence of condensable vapors. However, in the atmosphere such particles become rapidly more scarce with increasing size, and are relatively rare in sizes much larger than a few micrometers. At about 2 μm (0.00008 in.), coarse and fine particles are about equally abundant.

Using the term strictly, one rarely samples the atmospheric aerosol, but rather the particles out of the aerosol. The presence of aerosols is generally detected by their effect on light. Aerosols of a uniform particle size in the vicinity of the wavelengths of visible light can produce rather spectacular optical effects. In the laboratory, such aerosols can be produced by condensation of the heated vapors of certain oils on nuclei made by evaporating salts from heated filaments. If the suspending gas is cooled quickly, particle size is governed by the supply of vapor compared with the supply of nuclei, and the time available for condensation to occur. Since these can all be made nearly constant throughout the gas, the resulting particles are quite uniform. It is also possible to produce uniform particles by spraying a dilute solution of a soluble material, then evaporating the solvent. If the spray head is vibrated in an appropriate frequency range, the drops will be uniform in size, with the size controlled by the frequency of vibration and the rate of flow of the spray. Obviously, the final particle size is also a function of the concentration of the sprayed solution.

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Aerosols

Aerosols

Aerosols are collections of tiny particles of solid and/or liquid suspended in a gas. The size of these particles can range from about 0.001 to about 100 microns. While a number of naturally occurring aerosols exist, the most familiar form of an aerosol is the pressurized spray can. Aerosols are produced by a number of natural processes and are now manufactured in large quantities for a variety of commercial uses. They are also involved in a number of environmental problems, including air pollution and destruction of ozone in the atmosphere.

Aerosols are commonly classified into various subgroups based on the nature and size of the particles of which they are composed and, to some extent, the manner in which the aerosol is formed. Although relatively strict scientific definitions are available for each subgroup, these distinctions may become blurred in actual practical applications. The most important of these subgroups are the following:

Fumes

Fumes consist of solid particles ranging in size from 0.001 to 1 micron. Some typical fumes are those produced by the dispersion of carbon black, rosin, petroleum solids, and tobacco solids in air. Probably the most familiar form of a fume is smoke. Smoke is formed from the incomplete combustion of fuels such as coal, oil, or natural gas. Its particles are smaller than 10 microns in size.

Dusts

Dusts also contain solid particles suspended in a gas, usually air, but the particles are larger in size than those in a fume. They range from about 1 to 100 microns (and even larger) in size. Dust is formed by the release of materials such as soil and sand, fertilizers, coal dust, cement dust, pollen, and fly ash into the atmosphere. Because of their larger particle size, dusts tend to be more unstable and settle out more rapidly than do fumes, which do not settle out at all.

Mists

Mists are dispersions in a gas of liquid particles less than about 10 microns in size. The most common type of mist is that formed by tiny water droplets suspended in the air, as on a cool summer morning. If the concentration of liquid particles becomes high enough to affect visibility, it is then called a fog. A particular form of fog that has become significant in the last half century is smog, which forms when natural moisture in the air interacts with man-made components, such as smoke and other combustion products, to form chemically active materials.

Sprays

Sprays form when relatively large (10+ micron) droplets of a liquid are suspended in a gas. Sprays can be formed naturally, as along an ocean beach, but are also produced as the result of some human invention such as aerosol can dispensers of paints, deodorants, and other household products.

About three-quarters of all aerosols found in Earth’s atmosphere come from natural sources. The most important components are sea salt, soil and rock debris, products of volcanic emissions, smoke from forest fires, and solid and liquid particles formed by chemical reactions in the atmosphere. As an example of the last category, gaseous organic compounds released by plants are converted by solar energy in the atmosphere to liquid and solid compounds that may then become components of an aerosol. A number of nitrogen and sulfur compounds released into the atmosphere as the result of living and nonliving changes undergo similar transformations.

Volcanic eruptions are major, if highly irregular, sources of atmospheric aerosols. The eruptions of Mount Hudson in Chile in August 1991 and Mount Pinatubo in the Philippines in June 1991 produced huge volumes of aerosols that had measurable effects on Earth’s atmosphere.

Remaining atmospheric aerosols result from human actions. Some, such as the aerosols released from spray cans, go directly to form aerosols in the atmosphere. Others undergo chemical changes similar to those associated with natural products. For example, oxides of nitrogen and sulfur produced during the combustion of fossil fuels may be converted to liquid or solid nitrates and sulfates, which are then incorporated into atmospheric aerosols.

The physical and chemical properties of an aerosol depend to a large extent on the size of the particles that comprise it. When those particles are very large, they tend to have the same properties as a macroscopic (large size) sample of the same material. The smaller the particles, however, the more likely they are to take on new characteristics different from those of the same material in bulk. Aerosols tend to coagulate, or collide and combine with each other, to form larger bodies. A cloud, for example, consists of tiny droplets of water and tiny ice crystals. These particles move about randomly within the cloud, colliding with each other from time to time. As a result of a collision, two water particles may adhere (stick) to each other and form a larger, heavier particle. This process results in the formation of droplets of water or crystals of ice heavy enough to fall to Earth as rain, snow, or some other form of precipitation.

The synthetic production of aerosols for various commercial purposes has become such a large industry that the term aerosol itself has taken on a new meaning. Average citizens who know little or nothing about the scientific aspects of aerosols recognize the term as referring to devices for dispensing a wide variety of products.

Aerosol technology is relatively simple. A spray can is filled with a product to be delivered (such as paint), a propellant, and, sometimes, a carrier to help disperse the product. Pressing a button on the can releases a mixture of these components in the form of an aerosol.

The simplicity of this concept, however, masks some difficult technological problems involved in their manufacture. An aerosol pesticide, for example, must be formulated in such a way that a precise amount of poison is released, enough to kill pests, but not so much as to produce an environmental hazard. Similarly, a therapeutic spray such as a throat spray must deliver a carefully measured quantity of medication. In such cases, efforts must be taken to determine the optimal particle size and concentration in the aerosol by monitoring chlorofluorocarbon (CFC) propellants, which destroy the ozone layer.

The production of commercial aerosols fell slightly in the late 1980s because of concerns about the ozone and other environmental effects. By 1992, however, their manufacture had rebounded. In that year 990 million container units (bottles and cans) of personal aerosol products and 695 million container units of household products were manufactured. In the early 1990s many states passed legislation limiting the volatile organic compound (VOC) content in aerosols such as hairspray and spray paint. These limitations have forced the aerosol industry to seek alternate propellants and solvents. In many cases this substitution has resulted in inferior products from the standpoint of drying time and spray characteristics. The industry continued to struggle with these issues after the year 2000.

Combustion aerosols

Aerosol technology has made possible vastly improved combustion systems, such as those used in fossil-fueled power generator plants and in rocket

KEY TERMS

Acid rain— A form of precipitation that is significantly more acidic than neutral water, often produced as the result of industrial processes.

Chlorofluorocarbons (CFCs)— A group of organic compounds once used widely as propellants in commercial sprays, but outlawed in the United States in 1978 because of their harmful environmental effects.

Dust— An aerosol consisting of solid particles in the range of 1 to 100 microns suspended in a gas.

Electrostatic precipitator— A device for removing pollutants from a smokestack.

Fume— A type of aerosol consisting of solid particles in the range 0.001 to 1 micron suspended in a gas.

Mist— A type of aerosol consisting of droplets of liquid less than 10 microns in size suspended in a gas.

Ozone layer— A region of the upper atmosphere in which the concentration of ozone is significantly higher than in other parts of the atmosphere.

Smog— An aerosol form of air pollution produced when moisture in the air combines and reacts with the products of fossil fuel combustion.

Smoke— A form of smoke formed by the incomplete combustion of fossil fuels such as coal, oil, and natural gas.

Spray— A type of aerosol consisting of droplets of liquid greater than 10 microns in size suspended in a gas.

Stack gases— Gases released through a smokestack as the result of some power-generating or manufacturing process.

engines. The fundamental principle involved is that any solid or liquid fuel burns only at its surface. The combustion of a lump of coal proceeds relatively slowly because inner parts of the coal can not begin to burn until the outer layers are burned off first.

The combustion rate can be increased by dividing a lump of coal or a barrel of fuel oil into very small particles—the smaller the better. Power-generating plants today often run on coal that has been pulverized to a dust, or oil that has been converted to a mist. The dust or mist is then mixed with an oxidizing agent, such as air or pure oxygen, and fed into the combustion chamber. The combustion rate of such aerosols is many times greater than would be the case for coal or oil in bulk.

A number of environmental problems are associated with aerosols, the vast majority of them associated with aerosols produced by human activities. For example, smoke released during the incomplete combustion of fossil fuels results in the formation of at least two major types of aerosols that may be harmful to plant and animal life. One type consists of finely divided carbon released from unburned fuel. This soot can damage plants by coating their leaves and reducing their ability to carry out photosynthesis. It can also clog the alveoli (air sacs) in lungs, and interfere with respiration.

A second type of harmful aerosol is formed when stack gases, such as sulfur dioxide and nitrogen oxides, react with oxygen and water vapor in the air to form sulfuric and nitric acids, respectively. Mists containing these acids can be carried hundreds of miles from their source before conglomeration occurs and the acids fall to Earth as acid rain. Considerable disagreement exists about the precise nature and extent of the damage it causes, but there seems to be little doubt that in some locations it has caused severe harm to plant and aquatic life.

Ozone depletion

A particularly serious environmental effect of aerosol technology has been damage to Earth’s ozone layer. This damage appears to be caused by a group of compounds known as chlorofluorocarbons (CFCs) which, for more than a half century, were by far the most popular of all propellants used in aerosol cans.

Scientists originally felt little concern about the use of CFCs in aerosol products because they are highly stable compounds at conditions encountered on the Earth’s surface. They have since learned, however, that CFCs behave very differently when they diffuse into the upper atmosphere and are exposed to the intense solar radiation present there.

Under those circumstances, CFCs decompose and release chlorineatoms that, in turn, react with ozone in the stratosphere. The result of this sequence of events is that the concentration of ozone in portions of the atmosphere has been decreasing over at least the past decade, and probably for much longer. This change is not a purely academic concern, since the Earth’s ozone layer absorbs ultraviolet radiation from the sun and protects life on Earth from the harmful effects of that radiation. For these reasons, CFCs have been banned from consumer product aerosols since the late 1970s. They are still employed for certain medical applications, but by and large they have been eliminated from aerosol use. The aerosol industry has replaced CFCs with other propellants such as hydrocarbon gases (e.g., butane and propane), compressed gases (e.g., nitrogen and carbon dioxide), and hydrochlorofluorocarbons (which are much less damaging to the ozone layer.)

Technological solutions

Methods for reducing the harmful environmental effects of aerosols such as those described above have received the serious attention of scientists for many years. As a result, a number of techniques have been invented to reduce the aerosol components of things like stack gases. One device, the electrostatic precipitator, is based on the principle that the particles of which an aerosol consists (such as unburned carbon in stack gases) carry small electrical charges. By lining a smokestack with charged metal grids, the charged aerosol particles can be attracted to the grids and precipitated out of the emitted smoke.

Another risk associated with commercial aerosols is their use as recreational drugs. Deliberately inhaling of some consumer aerosol preparations may produce a wide variety of effects, including euphoria, excitement, delusions, and hallucinations. Repeated sniffing of aerosols can cause addiction intoxication, damaged vision, slurred speech, and diminished mental capacity.

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Aerosols

Climate Change: In Context
COPYRIGHT 2008 Gale

Aerosols

Introduction

Aerosols are naturally present microscopic airborne suspensions of liquid droplets or solid particles in a gas, usually air, which have tendencies to remain dispersed (floating) in the gas rather than to settle down. When the airborne suspensions are solid, such as dust and sea-salt, they are also commonly called particulate matter. A commonly seen natural event that spews out aerosols is an active volcano. Other examples of aerosols seen in Earth's atmosphere are mist, smog, and fog.

In addition, aerosols include all types of artificially made containers that hold a suspension of liquid or solid particles within a gaseous propellant under pressure. Such a container—sometimes called an aerosol dispenser, but more commonly called an aerosol can—seals in the suspension and, through a valve, dispenses it in a foam, liquid, or spray stream. Common products packaged in aerosol cans are cosmetics, detergents, foods, insecticides, and paints.

Historical Background and Scientific Foundations

The term aerosol is derived from the Greek term aero, which means pertaining to air, and the term sol, which is defined as any scattering, or dispersion, of microscopic or sub-microscopic particles in a liquid.

Naturally produced aerosol particles are considered by scientists to have a maximum size of one micrometer (or one millionth [10-6] of a meter). They generally range from 4 x 10-8 to 4 x 10-5 in (10-7 to 10-4 cm) in diameter. However, violently moving suspensions can contain aerosol particles that are over one-hundred times larger than

normal, as is often the case with fog and cloud droplets and dust particles.

The climate of Earth has been affected by artificially produced aerosols. The combustion of fossil fuels is one detrimental way that sulfate aerosols are introduced into Earth's atmosphere. Other sources of artificially produced aerosols include agriculture, deforestation, industry, mining, and transportation.

Artificially produced aerosols have been made from such particulate matter as asbestos, diesel fuel, and natural silicon dioxide (silica). However, these substances have been found to be harmful to humans, causing a variety of diseases such as anthracosis, a lung disease of coal miners commonly called black lung. It has been found that humans who work around such dangerous aerosols can be largely protected from health problems when they wear respirators.

Impacts and Issues

Aerosols are naturally found in Earth's troposphere and the stratosphere. The particular layer in which each aerosol type is found depends on its size, chemical composition, origin, and other such physical factors. These factors can determine whether aerosol particles absorb or scatter solar radiation and, thus, how much radiation reaches Earth's surface from the sun. Research is ongoing to learn more about the wide types of aerosols in the atmosphere because of their affect on global weather patterns and the climate in general.

WORDS TO KNOW

BIOMASS: The sum total of living and once-living matter contained within a given geographic area. Plant and animal materials that are used as fuel sources.

CONVECTION: The rising of warm air from an object, such as the surface of Earth.

DEFORESTATION: Those practices or processes that result in the change of forested lands to non-forest uses. This is often cited as one of the major causes of the enhanced greenhouse effect for two reasons: 1) the burning or decomposition of the wood releases carbon dioxide; and 2) trees that once removed carbon dioxide from the atmosphere in the process of photosynthesis are no longer present and contributing to carbon storage.

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

GLOBAL DIMMING: Decrease in amount of sunlight reaching Earth's surface caused by light blockage by clouds and aerosols. Global dimming increased from 1960 to 1990, reducing sunlight reaching Earth's surface by 4%, but this trend reversed after 1990 in most locations.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

PARTICULATE MATTER: Matter consisting of small particles. Particulate matter that is airborne forms aerosol pollution; particulate matter may also mix with water or lie on the surface of snow, ice, or ground. On snow or ice, small quantities of dark particulate matter (e.g., soot from fossil-fuel burning) can greatly accelerate melting.

PRECIPITATION: Moisture that falls from clouds. Although clouds appear to float in the sky, they are always falling, their water droplets slowly being pulled down by gravity. Because the water droplets are so small and light, it can take 21 days to fall 1,000 ft (305 m) and wind currents can easily interrupt their descent. Liquid water falls as rain or drizzle. All raindrops form around particles of salt or dust. (Some of this dust comes from tiny meteorites and even the tails of comets.) Water or ice droplets stick to these particles, then the drops attract more water and continue getting bigger until they are large enough to fall out of the cloud. Drizzle drops are smaller than raindrops. In many clouds, raindrops actually begin as tiny ice crystals that form when part or all of a cloud is below freezing. As the ice crystals fall inside the cloud, they may collide with water droplets that freeze onto them. The ice crystals continue to grow larger, until large enough to fall from the cloud. They pass through warm air, melt, and fall as raindrops.

STRATOSPHERE: The region of Earth's atmosphere ranging between about 9 and 30 mi (15 and 50 km) above Earth's surface.

SUSPENSION: A temporary mixture of a solid in a gas or liquid from which the solid will eventually settle out.

TROPOSPHERE: The lowest layer of Earth's atmosphere, ranging to an altitude of about 9 mi (15 km) above Earth's surface.

The study of aerosols in the atmosphere is important due to the number of artificially produced aerosols that have been, and continue to be, produced by humans. For instance, when humans burn fossil fuels, the aerosols produced by such activities absorb heat and, thus, contribute to global warming. Such aerosols are called atmosphere nuclei—that is, microscopic particles in the atmosphere that attract water droplets.

These aerosols tend to add more moisture to the atmosphere, which makes it more likely that precipitation will occur. Precipitation may occur outside of where the aerosols originated because it is easily transported by convection and wind currents. Consequently, problems around the world can occur due to localized aerosol use. Many of these atmosphere nuclei come from industrial environments where combustion forms particles such as sulfur dioxide and sulfur trioxide.

Some scientific research has found that many artificially produced aerosols have a cooling affect on the atmosphere. However, scientists contend, based on preliminary research, that such cooling (what is sometimes called global dimming) has a minimal effect on Earth's climate at best and does not counterbalance global warming, which is produced by greenhouse gases such as carbon dioxide, methane, and nitrous oxide.

Because of the increased presence of artificially produced aerosols in Earth's atmosphere, many organizations are actively seeking more and better information about aerosols. Almost continuously since 1978, the U.S. National Aeronautics and Space Administration (NASA) has provided aerosol maps of the world from observations made by a series of satellites called the Total Ozone Mapping Spectrometer (TOMS). The TOMS map provides a global map of aerosol particles found on any particular day on and above Earth. NASA also provides another map to show the cloud cover at the exact time the aerosol particle map is made.

The data from TOMS are being used by scientists around the world to better understand the behavior of aerosol particles within Earth's atmosphere. TOMS has been important to scientists because it was the first instrument to provide data of aerosol particles, especially as they move between land and water regions. Consequently, TOMS has provided valuable information about natural events such as forest fires and biomass burnings that affect the production of aerosol particles.

The TOMS satellites were replaced in January 1, 2006 by the Ozone Monitoring Instrument (OMI), which continues to provide even more advanced and detailed aerosol maps of the world as it orbits Earth onboard the Auro satellite, or Earth Observing Satellite (EOS) CH-1.

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Aerosols

The Gale Encyclopedia of Science
COPYRIGHT 2008 The Gale Group, Inc.

Aerosols

Aerosols are collections of tiny particles of solid and/or liquid suspended in a gas. The size of these particles can range from about 0.001 to about 100 microns. While a number of naturally occurring aerosols exist, the most familiar form of an aerosol is the pressurized spray can. Aerosols are produced by a number of natural processes and are now manufactured in large quantities for a variety of commercial uses. They are also involved in a number of environmental problems, including air pollution and destruction of ozone in the atmosphere.

Classification

Aerosols are commonly classified into various subgroups based on the nature and size of the particles of which they are composed and, to some extent, the manner in which the aerosol is formed. Although relatively strict scientific definitions are available for each subgroup, these distinctions may become blurred in actual practical applications. The most important of these subgroups are the following:

Fumes

Fumes consist of solid particles ranging in size from 0.001 to 1 micron. Some typical fumes are those produced by the dispersion of carbon black, rosin, petroleum solids, and tobacco solids in air. Probably the most familiar form of a fume is smoke. Smoke is formed from the incomplete combustion of fuels such as coal , oil, or natural gas . Its particles are smaller than 10 microns in size.

Dusts

Dusts also contain solid particles suspended in a gas, usually air, but the particles are larger in size than those in a fume. They range from about 1 to 100 microns (and even larger) in size. Dust is formed by the release of materials such as soil and sand , fertilizers , coal dust, cement dust, pollen, and fly ash into the atmosphere. Because of their larger particle size, dusts tend to be more unstable and settle out more rapidly than is the case with fumes, which do not settle out at all.

Mists

Mists are dispersions in a gas of liquid particles less than about 10 microns in size. The most common type of mist is that formed by tiny water droplets suspended in the air, as on a cool summer morning. If the concentration of liquid particles becomes high enough to affect visibility, it is then called a fog . A particular form of fog that has become significant in the last half century is smog . Smog forms when natural moisture in the air interacts with human-produced components, such as smoke and other combustion products, to form chemically active materials.

Sprays

Sprays form when relatively large (10+ microns) droplets of a liquid are suspended in a gas. Sprays can be formed naturally, as along an ocean beach, but are also produced as the result of some human invention such as aerosol can dispensers of paints, deodorants, and other household products.

Sources

About three-quarters of all aerosols found in the Earth's atmosphere come from natural sources. The most important of these natural components are sea salt , soil and rock debris, products of volcanic emissions, smoke from forest fires, and solid and liquid particles formed by chemical reactions in the atmosphere. As an example of the last category, gaseous organic compounds released by plants are converted by solar energy in the atmosphere to liquid and solid compounds that may then become components of an aerosol. A number of nitrogen and sulfur compounds released into the atmosphere as the result of living and non-living changes undergo similar transformations.

Volcanic eruptions are major, if highly irregular, sources of atmospheric aerosols. The eruptions of Mount Hudson in Chile in August 1991, and Mount Pinatubo in the Philippines in June 1991, produced huge volumes of aerosols that had measurable effects on Earth's atmosphere.

The remaining atmospheric aerosols result from human actions. Some, such as the aerosols released from spray-can products, go directly to form aerosols in the atmosphere. Others undergo chemical changes similar to those associated with natural products. For example, oxides of nitrogen and sulfur produced during the combustion of fossil fuels may be converted to liquid or solid nitrates and sulfates, which are then incorporated into atmospheric aerosols.

Physical properties

The physical and chemical properties of an aerosol depend to a large extent on the size of the particles that make it up. When those particles are very large, they tend to have the same properties as a macroscopic (large size) sample of the same material. The smaller the particles are, however, the more likely they are to take on new characteristics different from those of the same material in bulk. Aerosols tend to coagulate, or to collide and combine with each other to form larger bodies. A cloud, for example, consists of tiny droplets of water and tiny ice crystals. These particles move about randomly within the cloud, colliding with each other from time to time. As a result of a collision, two water particles may adhere (stick) to each other and form a larger, heavier particle. This process results in the formation of droplets of water or crystals of ice heavy enough to fall to Earth as rain, snow, or some other form of precipitation .

Synthetic production

The synthetic production of aerosols for various commercial purposes has become such a large industry that the term aerosol itself has taken on a new meaning. Average citizens who know little or nothing about the scientific aspects of aerosols recognize the term as referring to devices for dispensing a wide variety of products.

Aerosol technology is relatively simple in concept. A spray can is filled with a product to be delivered (such as paint), a propellant, and, sometimes, a carrier to help disperse the product. Pressing a button on the can releases a mixture of these components in the form of an aerosol.

The simplicity of this concept, however, masks some difficult technological problems involved in the manufacture of certain "spray" (aerosol) products. An aerosol pesticide, for example, must be formulated in such a way that a precise amount of poison is released, enough to kill pests , but not so much as to produce an environmental hazard. Similarly, a therapeutic spray such as a throat spray must deliver a carefully measured quantity of medication. In cases such as these, efforts must be taken to determine the optimal particle size and concentration in the aerosol by monitoring the CFC propellants, which destroy the ozone layer.

The production of commercial aerosols fell slightly in the late 1980s because of concerns about the ozone and other environmental effects. By 1992, however, their manufacture had rebounded. In that year 990 million container units (bottles and cans) of personal aerosol products and 695 million container units of household products were manufactured. In the early 1990s many states passed legislation limiting the volatile organic compounds (or VOCs) used in consumer product aerosols such as hairspray and spray paint. These limitations has forced the aerosol industry to seek alternate propellants and solvents. In many cases this substitution has resulted in inferior products from the standpoint of drying time and spray characteristics. The industry continued to struggle with these issues into the year 2000.

Combustion aerosols

Aerosol technology has made possible vastly improved combustion systems, such as those used in fossil-fueled power generator plants and in rocket engines. The fundamental principle involved is that any solid or liquid fuel burns only at its surface. The combustion of a lump of coal proceeds relatively slowly because inner parts of the coal can not begin to burn until the outer layers are burned off first.

The rate of combustion can be increased by dividing a lump of coal or a barrel of fuel oil into very small particles, the smaller the better. Power-generating plants today often run on coal that has been pulverized to a dust, or oil that has been converted to a mist. The dust or mist is then thoroughly mixed with an oxidizing agent, such as air or pure oxygen , and fed into the combustion chamber. The rate of combustion of such aerosols is many times greater than would be the case for coal or oil in bulk.

Environmental factors

A number of environmental problems are associated with aerosols, the vast majority of them associated with aerosols produced by human activities. For example, smoke released during the incomplete combustion of fossil fuels results in the formation of at least two major types of aerosols that may be harmful to plant and animal life. One type consists of finely divided carbon released from unburned fuel. This soot can damage plants by coating their leaves and reducing their ability to carry out photosynthesis . It can also clog the alveoli, air sacs in human lungs, and interfere with a person's respiration .

A second type of harmful aerosol is formed when stack gases, such as sulfur dioxide and nitrogen oxides, react with oxygen and water vapor in the air to form sulfuric and nitric acids, respectively. Mists containing these acids may be carried hundreds of miles from their original source before conglomeration occurs and the acids fall to Earth as "acid rain." Considerable disagreement exists about the precise nature and extent of the damage caused by acid rain . But there seems to be little doubt that in some locations it has caused severe harm to plant and aquatic life.

Ozone depletion

A particularly serious environmental effect of aerosol technology has been damage to the Earth's ozone layer. This damage appears to be caused by a group of compounds known as chlorofluorocarbons (CFCs) which, for more than a half century, were by far the most popular of all propellants used in aerosol cans.

Scientists originally felt little concern about the use of CFCs in aerosol products because they are highly stable compounds at conditions encountered on the Earth's surface. They have since learned, however, that CFCs behave very differently when they diffuse into the upper atmosphere and are exposed to the intense solar radiation present there.

In those circumstances, CFCs decompose and release chlorine atoms that, in turn, react with ozone in the stratosphere. The result of this sequence of events is that the concentration of ozone in portions of the atmosphere has been decreasing over at least the past decade, and probably for much longer. This change is not a purely academic concern since Earth's ozone layer absorbs ultraviolet radiation from the Sun and protects animals on Earth's surface from the harmful effects of that radiation. For these reasons, CFCs have been banned from consumer product aerosols since the late 1970s. They are still employed for certain medical applications, but by and large they have been eliminated from aerosol use. The aerosol industry has replaced CFCs with other propellants such as hydrocarbon gases (e.g., butane and propane), compressed gases (e.g., nitrogen and carbon dioxide ), and hydrochlorofluorocarbons (which are much less damaging to the ozone layer.)

Technological solutions

Methods for reducing the harmful environmental effects of aerosols such as those described above have received the serious attention of scientists for many years. As a result, a number of techniques have been invented for reducing the aerosol components of things like stack gases. One device, the electrostatic precipitator, is based on the principle that the particles of which an aerosol consists (such as unburned carbon in stack gases) carry small electrical charges. By lining a smokestack with charged metal grids, the charged aerosol particles can be attracted to the grids and precipitated out of the emitted smoke.

Aerosol sniffing

Another risk associated with commercial aerosols is their use as recreational drugs. Inhalation of some consumer aerosol preparations may produce a wide variety of effects, including euphoria, excitement, delusions, and hallucinations. Repeated sniffing of aerosols can result in addiction that can cause intoxication, damaged vision , slurred speech , and diminished mental capacity.

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